This invention relates to a straight tube Coriolis flowmeter and in particular to a case connect link that provides decreased thermal stress for a straight tube Coriolis flowmeter.
It is known that the elements internal to a straight tube Coriolis flowmeter are subject to stress when the flowmeter is subject to conditions in which temperature differentials are developed within a part or between various parts of the flowmeter. These temperature differentials can cause a part to expand/contract and either damage itself or other parts to which the expanding/contracting part is coupled. For example, the expansion/contraction of the flow tube by an amount different than the expansion/contraction of the flowmeter elements to which the flow tube is connected can stress the flow tube beyond the limits of its permissible deformation. Thus, if the flow tube expands an excessive amount with respect to the case and its end flanges, the flow tube may buckle. Conversely, if the flow tube contracts an excessive amount with respect to the case end flanges, it may develop cracks or tears or it may yield and be permanently deformed.
Attempts have been made to minimize problems associated with thermal contractions/expansion in flow tubes of Coriolis flowmeters. One solution utilizes materials having similar coefficients of expansion so that the expansion/contraction among all elements is uniform. Another solution uses a prestressed flow tube so it can change in length a modest amount without excessive internal stresses. Another solution uses bellows near the flow tube ends so the flow tube can change in length without undo stress. Another solution provides bends in the flow tube so that length changes are absorbed by the flow tube segment containing the bend. Another solution slidably mounts the ends of the flow tube to the case ends. These solutions reduce the problems associated with thermally induced length changes of the flow tube with respect to the elements of the flowmeter to which the flow tube is connected. However, these solutions have not solved the problems of thermally induced diameter changes in the flow tube. These diameter changes cause stresses in other Coriolis flowmeter elements including those that couple the flowmeter case to the vibratory elements of the flowmeter including the flow tube and its surrounding balance bar.
It is known to use case connect links to couple the vibratory end nodes of a flow tube to the flow tube case. This is done to prevent the end nodes from vibrating excessively during conditions of unbalance. Excessive vibration of the end nodes is undesirable because it causes changes in the flow sensitivity of the meter. Undesirable vibratory deflections at the brace bar (wherein the end nodes reside) is prevented by coupling the brace bar to a connecting link oriented transversely to both the vibratory direction of the flow tube and the axial direction of the flow tube. One end of the case connect link is connected to the case inner wall; the other end of the case connect link is connected to the brace bar or the end of the balance bar. The prior art case connect link is a relatively thin planar leaf spring member which is flexible in a twist mode and does not inhibit the rotation of the flow tube and balance bar at the desired vibration nodes. However, the case connect link prevents the end nodes from translating in the drive direction under conditions of unbalance. Each link prevents its respective end node from translating by rigidly coupling the end node to the relatively large mass of the case. The case connect link thereby improves the accuracy of the meter in measuring materials of higher or lower density than the nominal. A flowmeter having a case connect link of this type is shown in patent EP-0759542A1, published Feb. 26, 1997.
In Coriolis flowmeters of the straight tube type that employ case connect links, the flow tube extends for the entirety of the length of the flow meter with the flow tube ends being connected to case end flanges. The balance bar is typically parallel to the flow tube and may either be a separate parallel member or a cylindrical member surrounding the flow tube. The balance bar is shorter in length than the flow tube so that each end of the balance bar is connected by a brace bar to a near end portion of the flow tube. The brace bar typically is a relatively short circular member that extends transversely from the balance bar end to the near end location of the flow tube.
The vibratory node of the flow tube/brace bar combination normally resides in the brace bar during conditions of vibratory balance. Without case connect links, when materials of a relatively higher or relatively lower than nominal density flow in the flow tube, the vibratory node may move from the brace bar axially inward towards the flow tube center for heavy material or axially outward towards the end flanges for relatively light materials. These conditions of vibratory unbalance cause the brace bar to vibrate as part of the vibratory system and, in so doing, to shorten or lengthen the vibrating length of the flow tube to which it is connected. This change in length of the active portion of the flow tube is undesirable since it produces undesired changes in the flow sensitivity of the flow meter by altering the distance between the nodes and the pickoffs.
The use of case connect links positioned transversely with respect to the drive direction of the flow tube and the tube axis forces the vibration nodes of the flow tube/brace bar/balance bar combination to remain in the brace bar. The use of case connect links achieves the goal of minimizing undesired vibrations of the brace bar regions within the flow meter during conditions of vibratory unbalance associated with the processing of heavier or exceedingly light materials.
Although the use of case connect links minimizes undesired vibrations in straight tube Coriolis flowmeters, the case connect links are subject to structural damage when the temperature of the material in the flow tube differs from the case temperature. Under such conditions, the brace bar end of the case connect links may move in the radial direction a different distance than the case end due to different amounts of thermal expansion/contraction of the flow tube diameter. This results in the case connect links being subject to compressive or tensile loads that may stress and damage them or the elements to which they are connected.
It can therefore be seen that it is a problem to minimize structural damages to these case connect links resulting from differing amounts of thermal contractions/expansions of the case and the brace bar region.
It can be seen from the above that an aspect of the invention is a Coriolis flowmeter comprising:
a flow tube;
a balance bar oriented substantially parallel to the longitudinal axis of said flow tube;
first and second ends of said balance bar coupled to opposing near end portions of said flow tube;
a case containing said balance bar and said flow tube;
case connect link means coupling said first and second ends of said balance bar to an inner wall of said case;
at least one out of plane bend in said elongated case connect link means for enabling changes in the effective dimension of said case connect link means in response to thermal differences between said flow tube and said case.
Another aspect is that said flow tube is substantially straight.
Another aspect is that said balance bar is substantially cylindrical and surrounds said flow tube.
Another aspect is that said case is cylindrical and oriented substantially parallel to a longitudinal axis of said flow tube.
Another aspect is that said case connect link means is elongated and substantially flat and has a longitudinal axis oriented substantially perpendicular to the longitudinal axis of said flow tube and said balance bar;
first and second ends of said elongated case connect link means couple said first and second ends of said balance bar to an inner wall of said case; and
at least one out of plane bend in said elongated case connect link means that enables changes in the effective length of said elongated case connect link means in response to thermal differences between said flow tube and said case.
Another aspect is that said case connect means comprises:
a first and a second case connect link positioned at each end of said balance bar on opposite sides of said balance bar;
first and second ends on each of said case connect links that couple said first and second ends of said balance bar to said inner wall of said case.
Another aspect is that said case connect link means comprises a first and a second substantially circular diaphragm positioned at each end of said balance bar;
each diaphragm having a surface whose outer extremity couples said first and second ends of said balance bar to an inner wall of said case; and
at least one out of plane bend in said surface of said diaphragm that enables changes in the effective diameter of said diaphragm in response to thermal differences between said flow tube and said case.
Another aspect is that said case connect link means is elongated and has a surface that couples said first and second ends of said balance bar to an inner wall of said case; and
a plurality of out of plane bends in said surface of said elongated case connect link means that enable changes in the effective dimensions of said elongated case connect link means in response to thermal differences between said flow tube and said case.
Another aspect is that the flow tube has constant diameter for the entire length of the flowmeter.
Another aspect is that said case connect link means is elongated and has a bowed surface that enables said case connect means to change its effective length in response to thermal differences between said flow tube and said case;
ends of said bowed surface of said elongated case connect link means couple said first and second ends of said balance bar to an inner wall of said case.
Another aspect is that said case connect link means comprises a first case connect link means and a second case connect link means;
a first end of said case connect means couples one side of said inner wall of said case to a first side of said balance bar;
a second end of said case connect means couples an opposing side of said inner wall of said case to a second side of said balance bar; and
a middle portion of said case connect means defines a brace bar that couples said flow tube to said balance bar.
Another aspect is a Coriolis flowmeter comprising:
a straight flow tube;
a cylindrical balance bar surrounding said flow tube and oriented substantially parallel to the longitudinal axis of said flow tube;
first and second ends of said balance bar coupled to opposing near end portions of said flow tube;
a cylindrical case containing said balance bar and said flow tube and oriented substantially parallel to the longitudinal axis of said balance bar;
a first and a second circular diaphragm defining case connect link means coupled to each end of said balance bar and having a surface oriented substantially perpendicular to the longitudinal axis of said flow tube and balance bar;
the outer extremity of each said diaphragm couples said first and second ends of said balance bar to an inner wall of said case; and
at least one out of plane bend in said surface of each said diaphragm enables changes in the effective diameter of each said diaphragm in response to thermal differences between said flow tube and said case.
Another aspect is that said each said diaphragm has an inner portion that defines a brace bar connecting said flow tube and said balance bar.
Another aspect of the invention is a method of operating a Coriolis flowmeter having a flow tube adapted to receive a material flow and generate output information pertaining to said material flow; said flowmeter further having a balance bar oriented substantially parallel to the longitudinal axis of said flow tube, first and second ends of said balance bar coupled by a brace bar to opposing near end portions of said flow tube, a case containing said flow tube and said balance bar; and case connect link means coupling each end of said balance bar case to inner wall portions of said case; tube; said method comprising the steps of:
vibrating said balance bar and said flow tube in phase opposition during a material flow condition of said flowmeter to generate said output information pertaining to said flowing material; and
compensating said flowmeter for conditions of varying temperature differentials between said case and said flow tube by the provision of a surface of said case connect means containing an out of plane bend that facilitates changes in the effective dimensions of said case connect link means in response to said varying temperature differentials between said flow tube and said case.
The above problem is solved and an advance in the art is achieved in accordance with the present invention which provides a case connect link that has an out of plane bend which permits it to expand/contract in response to the presence of thermal expansion/contraction without permanent damage to the case connect link or the flowmeter elements to which it is connected.
In accordance with a first exemplary embodiment, each case connect link comprises a thin member that is elongated and which extends between the interior wall of the flowmeter case and a brace bar or end of the balance bar. The case connect link is positioned transversely with respect to the brace bar so that the elongated axis of the case connect link is transverse to the longitudinal axis of the flow tube and is also transverse to the drive direction of the flow tube. The case connect link of the present invention contains at least one out of plane bend in the portion of the link extending from the brace bar end to the case inner wall. Each out of plane bend comprises a fold that permits each half of the case connect link to expand or contract in the radial direction in response to thermal differentials with no permanent structural deformation. The out of plane bend may also comprise one or more sharp creases, a series of pleats like an accordion or alternatively may comprise a bowed or arc configuration.
In accordance with another possible embodiment of the invention, the case connect link may comprise a diaphragm having at least one pleat or the like with the circumference of the diaphragm coupled to the inner walls of the case. The center portion of the diaphragm is connected to the junction of the brace bar, the flow tube and the balance bar. The end portion of the flow tube extends through a hole in the center of the diaphragm and is connected to the case ends.
The case connect link provided in accordance with the present invention can accommodate thermal differentials and deform or contract substantially in response to radial expansion/contraction of the flow tube and balance bar with no damage to the material comprising the case connect link. This enables a flowmeter equipped with the case connect link of the present invention to process materials in an environment in which high temperature differentials can exist between different parts of the flowmeter without causing structural damage to the case connect link or the members to which they are connected.